1. Technical Field
The present invention relates to an area light source device that is used as a backlight device of a liquid crystal display device.
2. Background Art
An edge illuminated type area light source device is well known as the area light source device. The edge illuminated type area light source device includes an area light emitting section in which a diffusion plate and a prism sheet are stacked on a light guide plate and a light source that is disposed opposite an end face of the light guide plate. In the edge illuminated type area light source device, light emitted from the light source is introduced to the inside from the end face of the light guide plate, and the light diffused in the light guide plate is outputted from the diffusion plate and the prism sheet, thereby obtaining the area light emission.
For example, Patent Document 1 proposes an edge illuminated type area light source device in which a section shrinking portion whose thickness is decreased with distance from an incident end face is provided in an end portion of the light guide plate adjacent to the light source and the light incident from the end face is narrowed and propagated to an area light emitting region.
According to the technology disclosed in Patent Document 1, the thickness of the area light emitting region can be decreased in the area light source device to thin the liquid crystal display device. However, for part of the light emitted from the light source, an incident angle is decreased with respect to the inclined surface. As a result, part of the light leaks from the inclined surface. The leaking light is incident to the end face of the prism sheet or between the prism sheets, which generates uneven luminance in a liquid crystal panel (particularly, an edge portion near the light source).
Patent Document 2 proposes that ink is applied to the end face of the prism sheet. However, the even application of the ink to the end face of the extremely thin prism sheet (about 60 μm) significantly blocks quantity production of the prism sheet. Even if the ink can be applied to the prism sheet, reliability is extremely low, and it is not practical in consideration of the chip-off of the dried ink.    Patent Document 1: Japanese Unexamined Patent Publication No. 6-317797    Patent Document 2: Japanese Patent No. 3437029